From this site (link above) you may book your Northern Ireland driving theory test. A valid credit or debit card is required.
You may also Check, Change or Cancel your driving test appointment online provided you give three clear working days notice.
To use this service you will need to have your driver number, this can be found on your driving licence.
DVA advise you to take note of the appointment details or print the information in case you do not receive an appointment letter prior to the test date.